https://www.crowdsupply.com/protocentral/healthypi-move
This is a project I've been interested in for a bit. It looks like it will have a pretty good feature set out of the box, and with everything about it being open source I'm sure there will be an API for it at some point too. The price is a bit on the expensive side but is honestly pretty comparable to most current gen smartwatches tbh.
Yeah, it kinda sucks how cloud reliant most smart watches are. Like other people in the thread mentioned I think stuff like the pine time, bangle js, etc are probably your best bet for cheaper ones.
There's also asteroidOS which you can flash onto certain older smart watches. It's what I'm currently using but there isn't as much support for it so for more complicated data tracking would require setting up something like a raspberry pi to periodically fetch data over Bluetooth and process it yourself.